Today's electronic components generate significant amounts of heat which must be removed to maintain the component's junction temperature within safe operating limits. Failure to effectively conduct away heat leaves these devices at high operating temperatures, resulting in decreased performance and reliability and ultimately failure.
The heat removal process involves heat conduction between the electronic component and heat exchanger, or heat sink, via a thermal interface material. Small irregularities and surface asperities on both the component and heat sink surfaces create air gaps and therefore increase the resistance to the flow of heat. The thermal resistance of the interface between these two surfaces can be reduced by providing an interface material which fills the air gaps and voids in the surfaces.
An ideal medium for transferring heat from one surface to another should have low interfacial or contact thermal resistance, high bulk thermal conductivity and the ability to achieve a minimum bond-line thickness. Additional desirable qualities include product stability, ease of deployment, product reworkability, low cost and non-toxicity.
Liquids have low interfacial resistance because they wet a surface forming a continuous contact with a large area. Most liquids do not, however, have very high conductivity. Solids, and in particular metals, have very high conductivity but high interfacial resistance. Most common heat transfer materials combine highly conductive particles with a liquid or plastic in order to exploit both characteristics. Examples of the former are greases and gels while the latter include filled epoxies, silicones and acrylics.
Greases have been developed with thermal conductivities significantly better than the corresponding conductivities of filled adhesives. Typical problems with greases include pumping and dry out, both of which can cause the conducting medium to lose contact with one or both of the heat transfer surfaces. Pumping occurs when the structure is deformed, due to differential thermal expansion or mechanical loads, and the grease is extruded. The oils contained in a grease can be depleted by evaporation or by separation and capillary flow.
Liquid metal alloys (liquid at the operating temperature of the electronic component), such as alloys of bismuth, gallium and indium, potentially offer both low interfacial resistance and high conductivity. Several alloys of gallium with very low melting points have also been identified as potential liquid metal interface materials. Thermal performance of such an interface would be more than one order of magnitude greater than many adhesives typically in use.
Although liquid metal alloys offer both low interfacial resistance and high conductivity, they have historically suffered from various reliability issues including corrosion/oxidation, intermetallic formation, drip-out, dewetting, and migration. Unless mitigated, these mechanisms will continue to degrade the interface, resulting in a thermally related catastrophic failure of the actual electronic component.
The ability to contain an electrically conductive liquid within an electronic package presents significant challenges. The liquid must be reliably retained in the thermal interface throughout the life of the package if shorting is to be avoided and effective resistance is to be minimized. To solve the problems of liquid metal migration, various seal and gasket mechanisms have been disclosed.
Although, these various mechanisms mitigate liquid metal migration, some disclosures include elastomeric or polymeric components in the thermal path which is thermally undesirable. Other disclosures include various seals which increase the bondline thickness (BLT) of the liquid metal, thereby, increasing the bulk thermal resistance of the interface. These elastomeric components are not hermetic and therefore do not prevent air or moisture from entering the thermal joint.
In addition, corrosion will propagate through the thermal interface should any air gaps be present. Surface asperities of the heat source and heat exchanger increase the potential for voids. This is further exacerbated when the metal changes between the liquid and the solid state within the temperature range of the package.
U.S. Pat. No. 5,323,294 and 5,572,404, granted to Layton, et al. on Jun. 21, 1994 and Nov. 5, 1996, respectively, and U.S. Pat. No. 5,561,590, granted to Norell, et al. on Oct. 1, 1996 disclose a heat transfer module in which a compliant, absorbent body containing liquid metal is surrounded by a seal, said body is spaced apart from the seal area. As the module is clamped between a heat source and heat exchanger, liquid metal is squeezed out of the porous structure to fully fill the space within the seal area.
U.S. Pat. No. 4,915,167, granted to Altoz, et al. on Apr. 10, 1990 discloses a low melting point thermal interface material which is contained between the heat source and heat exchanger by applying a sealant to completely encapsulate the exposed interface material.
U.S. Pat. Nos. 6,761,928, 6,617,517, 6,372,997, granted to Hill, et al. on Jul. 13, 2004, Sep. 9, 2003, and Apr. 16, 2002, respectively, and U.S. Pat. No. 6,940,721, granted to Hill on Sep. 6, 2005 disclose a low melting point alloy coating both sides of a surface enhanced metallic foil, thereby providing a carrier to support and contain the liquid metal alloy. The low melt alloy on the foil carrier, placed between a heat source and heat exchanger, will become molten during the operational temperatures of the heat source.
U.S. Pat. No. 6,849,941, granted to Hill, et al. on Feb. 1, 2005 discloses a liquid metal interface material in which the material is bonded (in solid form) to a solid base member and includes a sealing material set into a annular groove (within the base member) surrounding the periphery of the bonded interface.
U.S. Pat. No. 6,037,658, granted to Brodsky, et al. on Mar. 14, 2000 discloses a heat transfer surface in which a thermally conductive fluid is contained by both an absorbent medium and a seal to inhibit migration.
U.S. Pat. No. 6,016,006, granted to Kolman, et al. on Jan. 18, 2000 discloses a method for applying thermal interface grease between an integrated circuit device and a cover plate in which a seal encloses the region of the device. Thermal grease is injected into the cavity region via an inlet port in the cover plate thereby filling the interface between device and plate.
U.S. Pat. No. 5,909,056, granted to Mertol on Jun. 1, 1999 discloses a thermal interface structure in which a phase change thermal interface material is contained within a protrusion on a heat spreader and a dam ring, which is attached to the backside of a semiconductor chip.
U.S. Pat. No. 6,891,259, granted to Im, et al. on May 10, 2005 and U.S. patent application No. 20030085475, filed by Im, et al. on Oct. 10, 2002 disclose a semiconductor package in which a dam substantially surrounds the thermal interface material. The package lid includes injection holes for the dispensation of the dam and interface material.
U.S. Pat. No. 6,292,362, granted to O'Neal, et al. on Sep. 18, 2001 discloses a thermal interface material module in which a flowable interface material is deposited in the center opening of a picture-frame carrier and a gasket is mounted to the carrier. With the application of heat, the reservoir area between the interface material and gasket is filled.
U.S. Pat. No. 6,097,602, granted to Witchger on Aug. 1, 2000 discloses a thermal interface structure in which a phase change interface material is surrounded by a fabric carrier dike structure. The dike is adhesively attached to both the electronic circuit package and heat sink, thereby preventing interface material from migrating from the joint.
U.S. Pat. Nos. 6,281,573 and 6,656,770, granted to Atwood, et al. on Aug. 28, 2001 and Dec. 2, 2003, respectively, disclose both a solder-based seal (between the ceramic cap/heat exchanger and package substrate) and an elastomeric gasket (between the ceramic cap/heat exchanger and chip) to “near hermetically” seal the cavity containing a Gallium alloy liquid metal interface material and thereby limit oxidation and migration.
U.S. Pat. No. 6,665,186, granted to Calmidi, et al. on Dec. 16, 2003 discloses a liquid metal interface material held in place by a flexible seal, such as an O-ring, which also accommodates expansion and contraction of the liquid. The seal also allows for air venting and filling of liquid metal.
U.S. patent application No. 20030173051, filed by Rinella, et al. on Mar. 12, 2002 discloses a method of forming a thermal interface in which a semi-solid metal, injected through an inlet on a heat spreader plate, fills the gap between a die and the cavity formed in the heat spreader plate.
U.S. patent application No. 20030183909, filed by Chiu on Mar. 27, 2002 discloses a method of forming a thermal interface in which a thermal interface material is dispensed through and inlet in a heat spreader in order to fill the interface between the spreader and the chip.
U.S. patent application No. 20040262766, filed by Houle on Jun. 27, 2003 discloses a liquid metal interface contained within a cold-formed o-ring barrier positioned directly on the chip. Once the barrier is established between the heat spreader and chip, liquid metal is introduced into the interface via a channel in the spreader.
U.S. patent application No. 20050073816, filed by Hill on Jan. 7, 2004 discloses a liquid metal interface assembly in which an o-ring or shim sealing member surrounds the liquid metal interface material to shield the interface from the atmosphere.
FIGS. 1 through 3 show various methods of forming a void-free, high thermal performance thermal interface within electronic assemblies 100. FIG. 1A illustrates an electronic assembly 100 comprised of a thermal interface structure 102 positioned between a heat spreader lid 104 and electronic component 106, which is comprised of an IC chip 108, package substrate 110 and electrical interconnection vias 112. The interface structure 102 is comprised of a metallic core 120 encapsulated by a metallic interface composition 122. An adhesive layer 114 bonds the heat spreader lid 104 to the electronic component package substrate 110. It can be seen in FIG. 1B that the lid 104 has now been mounted to the package substrate 110 with an adhesive layer 114 located on the lid flange 116. During operation of the electronic component 106, the resultant heat will cause excess metallic interface composition 122 to flow out of the thermal interface, thereby creating a fillet 118 outside the IC chip perimeter. Unfortunately, oxidation, present on the surface of the metallic interface 122 prior to heating and flowing, creates a “skin” and prohibits filling of the surface asperities present on the lid 104 and IC chip 108. FIG. 1c, a magnified sectional view of FIGS. 1a and 1b, illustrates the resultant air gaps 123 due to the layer of oxidation 125 inhibiting flow of interface material. The non-hermetic interface allows oxygen and moisture to penetrate into these air gaps 123 and continue oxidation/corrosion of the metallic interface composition 122 within the interface between chip 108 and lid 104.
Within FIG. 2, it can be seen that a metallic thermal interface composition is injected (by a dispenser 124) through a hole 126 in the heat spreader lid 104 to yield a filled thermal interface joint 128. Without a barrier or seal, interface material would have the tendency to migrate out of the joint. The use of a seal will promote full filling of the thermal joint as well. Additionally, the hole 126, filled with the interface composition would certainly possess lower thermal conductivity than the typical materials (copper, aluminum) comprising heat spreader lids.
FIG. 3, similar to FIG. 2, illustrates an electronic assembly 100 comprised of a thermal interface structure 130 sandwiched between an IC chip 108 and heat spreader lid 104. The lid 104 includes at least one gas permeable plug 132 located within holes 134 in the lid 104. A barrier or seal 136 is placed near the perimeter of the IC chip 108, thereby creating a seal and space between the lid 104 and IC chip 108. Liquid interface material 138 is injected into the holes 134 in the lid 104, thereby filling the space comprising the thermal interface joint. Should the barrier be of polymeric composition, heat transfer would be reduced near the perimeter of the chip. A metallic barrier would require a bonding and hermetic seal in order for the gas permeable plugs to be effective. Barrier bonding may induce unwanted stresses between the IC chip 108 and the lid 104. Additionally, the holes 134 in the lid would also created undesirable thermal impedance between the chip 108 and lid 104.